Why does a pond freeze from the top up?

A pond freezes from the top down primarily due to the unique properties of water, particularly its density. As water cools, it becomes denser until it reaches about 4°C; below this temperature, it starts to expand. This expansion causes ice to form on the surface first, as it is less dense than liquid water, creating an insulating layer that prevents the colder air from affecting the water below. Consequently, the ice forms on top, allowing life beneath to continue in the liquid water.